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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

482                               ALASEAN GLACIER STUDIES
fiords. The inner fiord portions of the two inlets present no fundamentally important difference excepting that while the fiord portion of the Yakutat Bay inlet is confined to a single fiord only moderately branched, the corresponding portions of Prince William Sound are numerous and some of the fiord fingers are much branched. Aside from this difference the fiords of the two regions are so similar that they can be characterized in the same general terms, as narrow, mountain-walled fiords, with steep sides, truncated spurs, hanging valleys, and every indication of powerful glacial erosion as the chief agent in their shaping. They resemble also the fiords of the Inside Passage in southeastern Alaska.
Here, however, the resemblance between Yakutat Bay and Prince William Sound ends. In the former the single, moderately-branching fiord grades into a broadly-open, outer bay whose margins are formed not by mountains, but by a foreland of outwash gravel on one side and an ice plateau with a fringe of gravels on the other. But in Prince William Sound a dozen fiords from north, east, and west terminate in a roughly rectangular inlet, mountain-walled on three sides and on the other, or ocean side, defended from the ocean waves by mountainous islands.
Extent of Former Glaciation. At an earlier period, certainly at least several centuries ago, Yakutat Bay was completely filled with ice and a great Yakutat Bay Glacier discharged icebergs directly into the Pacific. From the evidence of the great amount of erosion performed and of the extensive deposits made it is clear that this period of extensive glaciation was of long duration. Prince William Sound, also, at an earlier period, and presumably at the same period as in Yakutat Bay, was occupied by an extensive glacier.
During the period of greatest glaciation, the branching tributary fiords of Prince William Sound were well filled with great glaciers and these, converging from all sides, coalesced in the broad sound to form an expanded ice sheet of the piedmont type, (PI. CLXXXTV) which crowded up against the islands which lie across the mouth of the sound, crept out of the openings between them, and rose above the tops of all buts the westernmost islands.
Origin of the Two Inlets. The fiord portions of both inlets as already intimated, are interpreted as the product of profound glacial erosion. The phenomena of glacial erosion are clear and convincing in Prince William Sound, as they are in the fiords of the Yakutat Bay inlet. Long-continued, vigorous ice motion along these fiords alone seems capable of explaining these phenomena.
With the outer bays the case is quite different. Outer Yakutat Bay, though doubtless modified in the northern portion by glacial erosion, and in the outer portion by glacial deposition, has not, in the main, the features which result from shaping by glacial erosion. We believe that this outer bay is a part of the shallow ocean bed on the continental shelf, protected by glacial occupation and deepened slightly by glacial erosion while the border of glacial outwash gravels was being laid down on the eastern side. What will be the result when the Malaspina Glacier disappears from the western side cannot be stated, but it seems probable that a port of its bed will become a bay of similar origin.
The origin of the main Prince William Sound is wholly different from this. It is a rock-enclosed bay. We are not in a position to state its origin with absolute certainty, though there are some facts of significance, pointing toward origin by glacial erosion. At first thought this may seem a very far-fetched hypothesis and one contrary to probabilities,